It is known that watercraft, such as surface vessels, submarines, and such like, can be detected and classified by means of the operating noises they emit. For this purpose, these operating noises which are mainly caused by the prime movers, are received using a sonar system and related received signals are generated. Subsequently, the received signals are transformed into a frequency spectrum which is analyzed according to an absolute-value generation with respect to frequency lines and corresponding frequencies. Due to the driving propellers, turbines, generators and/or pumps, significant frequency lines are present in the amplitude spectrum generated from the frequency spectrum of the received signal.
DE 42 204 29 A1 shows a method for the detection and classification of a propeller-driven vessel on the basis of fuzzy logic. In this method, association functions are defined when it appears plausible that a spectral line exists in the frequency spectrum, and this spectral line does not belong to the background noise, but is a part of a band of harmonics or its fundamental frequency itself. The disadvantage here, however, is the complex fine-tuning required for the control of the system. The association function of the fuzzy set must thus be rectified in complex fine-tuning work. The disadvantage in this case is that only frequency lines or their harmonics are considered as fundamental frequency in this classification method.